Production of granular alkali metal tripolyphosphate of high abrasion resistance and high apparent density

ABSTRACT

Production of granular alkali metal tripolyphosphate of high abrasion resistance and high apparent density. An alkali metal tripolyphosphate of which between 10 and 40 percent of its particles have a size smaller than 0.03 mm, the balance particles having a size of between 0.03 and 0.4 mm, is granulated by spraying an alkali metal orthophosphate solution thereonto; the resulting granules are dried; undersize and oversize particle fractions are separated from the dried granules; and the remaining fraction of particles is annealed at temperatures of between 300 and 600*C.

United States Patent [191 Hinz 'et al.

PRODUCTION OF GRANULAR ALKALI METAL TRIPOLYPHOSPI-IATE OF HIGH ABRASIONRESISTANCE AND HIGH APPARENT DENSITY Inventors: Arnulf I-Iinz, Knapsack;Heinz Harnisch, Lovenich, both of Germany Assignee: KnapsackAktiengesellschaft,

Knapsack near Cologne, Germany Filed: Feb. 9, 1971 Appl. No.: 114,048

Foreign Application Priority Data Feb. 24, 1970 Germany P 20 08 495.1

References Cited UNITED STATES PATENTS 12/1964 Metcalf et a1 23/106Sept. 25, 1973 3,210,154 10/1965 Klein et al. 23/107 X 3,437,433 4/1969Sproul et a1. .t

3,437,434 4/1969 Sproul et a1. 23/106 FOREIGN PATENTS OR APPLICATIONS234,197 12/1924 Great Britain 23/107 1,536,511 8/1968 France 23/107Primary Examiner-0scar R. Vertiz Assistant Examiner-Gregory A. HellerAttorneyConnolly and Hutz [5 7] ABSTRACT Production of granularalkalimetal tripolyphosphate of high abrasion resistance and high apparentdensity. An alkali metal tripolyphosphate of which between 10 and 40percent of its particles have a size smaller than 0.03 mm, the balanceparticles having a size of between 0.03 and 0.4 mm, is granulated bysprayingan alkali metal orthophosphate solution-thereonto; the resultinggranules are dried; undersize and oversize particle fractions areseparated from the dried granules; and the remaining fraction ofparticles is annealed at temperatures of between 300 and 600C.

9 Claims, No Drawings PRODUCTION OF GRANULAR ALKALI METALTRIPOLYPHOSPI'IATE OF HIGH ABRASION RESISTANCE AND HIGH APPARENT DENSITYThe present invention relates to the production of granular alkali metaltripolyphosphate of high abrasion resistance and high apparent density,preferably higher than 750 g/liter.

Granular, non-dusting alkali metal tripolyphosphate, which shouldsubstantially resist abrasion during transport or on working it into ablend, is gaining increasing interest in industry for the production ofdetergents and rinsing agents. Undersize particles, which arenecessarily obtained in the production of granular, non-dusting alkalimetal tripolyphosphate are accordingly required to be screened off.Unless use is made thereof in special detergents, the undersize materialis required to be redissolved and resprayed, or to be used as returnmaterial, conditional upon the process employed in a given case.

German published Specification l 442 992 describes the production ofcoarsely granular sodium tripolyphosphates of low apparent density,wherein fine particulate sodium orthophosphate of which more than 90percent of its particles have a size smaller than 0.3 mm in diameter andmore than 50 percent of its particles have a size smaller than 0.1 mm indiameter, is granulated by spraying an aqueous alkali metalorthophosphate solution thereonto, and the resulting spray product isheated to temperatures of between 250 and 550C. The spray water is usedin a proportion of between 4 and 30 percent and the solution sprayed inthe form of droplets having a size of at most 03 mm in diameter. i

The spray product consists of hollow beads and is required to be treatedunder very mild conditions. While on the one hand it is desirable thatthe product be kept in motion on heating'it to temperatures of between250 and 550C it is necessary on the other hand to avoid the occurrenceof abrasion, pressure and squeezing effects. These, however, areconditions which are very difficult to realize in a commercial process.Granules of low apparent density, even those which are sucessivelyannealed, are commonly known to have a minor abrasion resistance onlyand this in combination with a limited stability during transport, whichis a result of their hollow bead structure and which is sodisadvantageous, particularly on subjecting the granules to finishingtreatment. This is the reason why products of high abrasion resistanceare gaining increasing interpolyphosphate of high apparent density andhigh abra-.

sion resistance from fine particulate alkali metal tripolyphosphate.This process accordingly enables beneficial use to be made of the aboveundersize particles of which the uses have been very limited heretofore.

The process of the present invention comprises more particularlygranulating an alkali metal tripolyphosphate of which between 10 and 40percent of its particles have a size smaller than 0.03 mm, the balanceparticles having'a size of between 0.03 and 0.4 mm, by spraying analkali metal orthophosphate solution thereontoydrying the resultinggranules; separating undersize and oversize fractions from the driedgranules; and annealing the remaining fraction of particles by heatingthem to temperatures of between 300 and 600C. The alkali metaltripolyphosphate used as the starting material in the process shouldpreferably consist of particles of which between 20 and 30 percent havea size smaller than 0.03 mm, and the balance particles have a size ofbetween 0.03 and 0.4 mm.

The starting material should preferably be granulated with the use of analkali metal orthophosphate solution which contains alkali metal oxide P0 in a ratio of substantially 1.66, and may also be used for theproduction of alkali metal tripolyphosphates.

These solutions substantially contain 50 weight percent of dissolvedphosphate and are used at a temperature of approximately C.

Following separation of undersize and oversize particles from theparticle fraction that is desired to be produced, the oversize particlesare preferably crushed, particles of desirable size are separatedtherefrom once again, and the two fractions of undersize particles arecombined with starting material.

By regulating the annealing period and temperature it is possible toestablish a given concentration of phase-l or phase-II in the finalproduct. For example, a product containing between 50 and percent ofphase-l is obtained by effecting the annealing step over a period ofbetween about l5 and 30 minutes at temperatures of between about 520 and550C, and a product containing between 0 and 50 percent of phase-l isobtained by effecting the annealing step over a period of between about10 and 30 minutes at temperatures of between about 300 and 500C.

The phase concentration should preferably be stabilized by quenching thehot granules and cooling them down to room temperature.

One of the beneficial effects of the process of the present inventionresides in the fact that between 10 and 40 percent of the particles havea fineness of less than 0.03 mm. In other words, the granules thensubstantially fail to have a hollow bead structure and combine this withhigher density.

A further beneficial effect resides in the fact that the spray productremains for a short while only on the granulating disc and accordinglyfails to be very closely packed. In other words, the spray productinitially is still in the state of plasticity, when delivered to arotary tube. By intense rotation therein, it is further strengthened andfinally has an abrasion resistance greatly excelling that of hollowbeads.

A still further beneficial effect resides in the fact that following thedrying and screening steps the resulting product is annealed in aseparate annealing tube of which the speed of rotation, angle ofinclination and degree of filling can be so modified, independently ofthe drying tube, that a final product of optimum abrasion resistance isproduced.

In carrying out the process of the present invention it is necessary touse the tripolyphosphate in the form of particles of which between 10and 40 percent have a size smaller than 0.03 mm. If use is made ofmaterial of which less than 10 percent of its particles have a sizesmaller than 0.03 mm, the gaps left between the individual particlesremain unfilled for lack of fines, and

loosely agglomerated particles, which are merely adhered together by thealkali metal orthophosphate spray solution, are obtained. Particulatematerial having a size larger than that of the fines, which have aparticle size of less than 0.03 mm, has a smaller wettable surface, anda substantially smaller proportion of alkali metal phosphate bindersolution can accordingly be added thereto until granulation with theresult that, while the throughput of a granulating disc is greatlyincreased for the use of a given nozzle, the agglomerates practicallycease to undergo further strengthening in the rotating tube. Theselatter accordingly have a very minor resistance to abrasion. The reasonfor this is that the crystals originating from the alkali metalphosphate solution are unable to form firm bridges between theindividual particles.

If use is made of material of which more than 40 percent of itsparticles have a size of less than 0.03 mm, there is an excess of fines,and coarser granules of high density and high abrasion resistance arenecessarily produced. lf use is made of material containing 40 percentor more of fines, with a size of less than 0.03 mm, then the largersurface of these particles dictates the use of a larger proportion ofalkali metal phosphate solution as the binder. As a result of this, thethroughput on the disc is diminished for the use of a given nozzle, andoversize granules are required to be crushed, with the resultantformation of 50 percent or more of return material having a size of lessthan 0.4 mm, for example. While the crushed particles have a highabrasion resistance, the fact remains that the'large proportion ofundersize return material is unacceptable from an economical point ofview. Optimum yields of desirable primary particles having a size ofbetween 0.4 and 1.5 mm, for example, are produced by granulation ofmaterial of which between 20 and 30 percent of its particles have a sizeof less than 0.03 mm. The yield normally is between 55 and 65 percentand partially even higher. By subjecting the granules to drying,successively crushing oversize material (between and 30 percent with asize larger than 1.5 mm) and recovering so-called secondary particles",it is even possible to produce a total yield of desirable particles ofbetween 70 and 80 percent.

Undersize particles are used as return material and directly added tothe starting material. This effects improved wettability duringgranulation and more uniform formation of granules.

A rotary disc should preferably be used as the granulating means. Ascompared with a double shaft mixer or rotary drum, for example, therotary disc enables the granulation process to be continuously monitoredand corrected, if necessary. If use is made of a double shaft mixer,then it is impossible satisfactorily to influence the size of theresulting granules, for example by varying the speed of rotation orposition of the paddles. In addition thereto, material which cakestogether on the paddles and is dropped off from time to time effects theformation of coarse lumps. These are merely superficially dried in therotating tube and they agglutinate the crushing means. If use is made ofa rotary drum, the granulation process again is not easy to monitor andcontrol, especially in those cases in which the product issimultaneously dried therein. I

On granulating the particulate material by spraying water thereonto bymeans of a single material nozzle, the particles are more or lessrapidly hydrated, this conditional upon the phase concentration in thematerial to undergo granulation, with the result that the granules arepartially subjected to break-up. To attain complete granulation, itwould accordingly be necessary, this conditional upon the proportion ofparticles with a size of less than 0.03 mm, to use between 33 and 38percent of water, based on the dry substance, and later again to removethe water.

We have further discovered in accordance with our present invention thatconsiderable beneficial effects are produced by replacing the waterbinder by an alkali metal phosphate solution having a temperature ofapproximately C and being normally used for the production oftripolyphosphate in a spray tower. For a specific gravity ofapproximately 1.590 at 90C, such alkali metal phosphate solutioncontains substantially 50 weight percent of dissolved substance, basedon tripolyphosphate, and contains monoalkali metal phosphate dialkalimetal phosphate in the ratio of 34.4:66.6, corresponding to an alkalimetal oxide P O -ratio of 1.66:].

Upon spraying such phosphate solution, impinging contact is producedbetween the individual droplets of the solution and the stream of powderrotating on the disc, and small granules are produced at once. At thesame time, the alkali metal orthophosphate contained in each of theindividual droplets commences to crystallize therefrom, and the granulesare given a firm structure which is further strengthened until removalof the granules from the disc, and this without any need for thetripolyphosphate, that is the real binder, superficially to dissolve inthe moist granules, as in water granulation. Between about 30 and 40weight percent of alkali metal phosphate solution, based on groundtripolyphosphate, is required to be used in the granulation process ofthe present invention, conditional upon the fineness of the particles toundergo granulation.

In other words, for a concentration of approximately 50 weight percentof substance dissolved in the solution, merely about half as much wateris introduced, based on the granulation with water, and thetripolyphosphate is accordingly subject to partial hydration only. Therapid crystallization of orthophosphate from the droplets binding theground solid material also produces beneficial effects, which reside inan increased formation of primary particles having a size of between 0.4and 1.5 mm, for example, based on granulation with water, and in theformation of oversize particles having a size of between 2.0 and at most2.5mm, compared with their normal size of 4 mm in diameter.

As compared with the use of water as the granulating agent, the use ofan alkali metal phosphate solution as the binder additionally enablesthe output of desirable particles to be increased at a rate of between10 and 20 percent, based on the fine particulate tripolyphosphate. Afurther advantage resides in the fact that the very same alkali metalphosphate solution can be used, and this in the absence of anyadditional preparatory treatment, for the production oftripolyphosphate, for example in a spray tower, and as a binder in thegranulation process.

While alkali metal phosphate solutions of analogous composition but withhigher proportions of dissolved material therein and higher specificgravities, for example 1.645, are indeed easy to spray and even improvethe abrasion resistance of the resulting granules, the fact remains thatthe granules contain more orthophosphate and are therefore required tobe left in the rotating tube for longer periods of time. In additionthereto, it is impossible to use such solutions in a spray tower. Thesteps of drying and annealing the moist granules and the step ofsimultaneously establishing the phase concentration should convenientlynot be effected in a single rotary tube. The reason for this is thatuniform phase standardization in all of the individual particlefractions is rendered impossible by the different size of thoseparticles.

The granules discharged from the rotary disc should therefore be driedin a rotating tube, oversize material screened off and crushed, andmerely the particle fraction of desirable size, for example that of asize of between 0.4 and 1.5 mm, annealed in a second rotating tube. Itis possible in this way to modify the speed of rotation and angles ofinclination of the tubes as well as the temperatures and residence timesin the two tubes, independently of one another.

The abrasion resistance can be determined by two different methods. Inone of these, the drum test, 50 grams of granules having a size ofbetween 0.4 and 0.8 mm are rolled for 5 minutes and togetherwith 8 steelballs of 20 mm in diameter in a small cylindrical drum. The drum has adiameter of 114 mm, a depth of 100 mm, and rotates at a speed of 100rpm. The material retained on' a 0.4 mm test sieve, expressed in weightpercent, is an index of the abrasion resistance.

In the sieve test, 100 grams of granules having a size of between 0.4and 0.8 mm are placed together with 35 steel balls with a diameter of 15mm and a total weight of 495 grams on a 0.4 min test sieve of 200 mm indiameter, and horizontally yet eccentrically vibrated thereon for a 10minute period at 160 rpm and a 30 mm breadth of oscillation. Here again,the material retained on sieve, expressed in weight percent, in an indexof the resistance to abrasion.

The rate of hydration is determined by the ROH-test (rate of hydrationtest). 200 milliliters of water are placed in a Dewar vessel, heatedtherein to a temperature higher than 80C. and 50 grams of Na,SO,, areadded. The salt is allowed to dissolve and the whole is cooled. As soonas the cooling curve transverse the 80C line, there are added 150 gramsof tripolyphosphate and the temperature increase is measured. A productis qualified as good when a temperature of between 89 and 90C is reachedafter 1 minute.

EXAMPLE 1 Y The starting material was fine particulate sodiumtripolyphosphate containing 30 percent of return material, and thebinder was a sodium phosphate solution which had a specific gravity of1.595 at 90C and contained Nap/P 0 in the molar ratio of 1.66. Structureof particles:

a. Sodium tripolyphosphate free from return material b. Sodiumtripolyphosphate with 30 percent of return material Retained on sieve:

a) 0.3 mm 1.2 weight percent 6.3 weight percent 0.2 mm 7.6 weightpercent 20.6 weight percent 0.1 mm 37.7 weight percent 43.6 weightpercent 0.06 mm 50.5 weight percent 60.2 weight percent 0.03 mm 68.8weight percent 82.0 weight percent 0.03 mm 31.2 weight percent 18.0weight percent The granulation was effected on a rotary disc, which hada diameter of 1,000 mm, was inclined through an angle of 62, withrespect of the horizontal, and rotated at a speed of 22 rpm.

The phosphate solution was sprayed under a pressure of 3 atmospheresgauge by means of a single material spray nozzle with an orifice of 1.6mm in diameter.

The throughput was 1.306 kg/min. 0.815 l/min.

The operation period was 6 hours and 49 minutes. 1.416 kg oftripolyphosphate with 30 percent of return material and 534 kg ofNa-phosphate solution were put through, and 1.950 kg of moist granules,corresponding to 286 kg/hour, were obtained.

Based on the sodium tripolyphosphate, there were needed 37.7 weightpercent of sodium phosphate solution, from which a further 18.35 weightpercent of sodium tripolyphosphate was formed, corresponding to theproportion of solid matter dissolved therein.

The moist granules discharged from the rotary disc were dried in aheated rotary tube which had a diameter of 500 mm and a length of 2 000mm, rotated at a speed of 12 rpm and was not inclined. The granulationtemperature near the discharge end was between 180 and 200C.

Particles with a size of between 0.4 and 1.5 mm (primary particles) weresieved out for calcination. Oversize particles were crushed in a hammermill and particles with a size of between 0.4 and 1.5 mm (secondaryparticles) were sieved out. The undersize particles with a size of lessthan 0.4 mm, which were obtained in the two cases, were used as returnmaterial and added to the starting material.

. Primary particles were obtained in .a yield of 57 weight percent. Withthe inclusion of the secondary particles the total yield was 68 weightpercent, based on the starting material.

Calcination and phase standardization were effected in a second rotarytube which rotated at a speed of 8 rpm, had dimensions the same as thoseof the drying tube, and was fitted with inserts to ensure thoroughmixing and uniform distribution of the temperature in the rotatinggranules. The granules were allowed to remain therein over a period of20 minutes and then had a temperature of 540C.

The hot granules were quenched and successively cooled down to roomtemperature, in a cooling tube.

The finished granules had the following properties:

Apparent density: 890 grams/liter.

EXAMPLE 2 The starting material was fine particulate sodiumtripolyphosphate containing 30 weight percent of return material, andthe binder was a sodium orthophosphate solution which had a specificgravity of 1.620 at C and contained Na O/P,O in the molar ratio of 1.66.Structure of particles:

a. Sodium tripolyphosphate b. Sodium tripolyphosphate with 30 percentreturn material.

Retained on sieve:

0.3 mm 0.5 weight percent 12.4 weight percent 0.2 mm 3.9 weight percent26.3 weight percent 0.1 mm 25.2 weight percent 45.1 weight percent 0.06mm 36.0 weight percent 55.0 weight percent 0.03 mm 63.0 weight percent74.7 weight percent 0.03 mm 37.0 weight percent 25.3 weight percent Thegranulating disc and nozzle were the same as those used in Example 1.

The throughput rate was 1.215 kg/minute 0.750 liter/minute, and theoperation period was 6 hours and 42 minutes.

1.276 kg of sodium tripolyphosphate containing 30 percent of returnmaterial and 488 kg of sodium phosphate solution were put through withinthis period, and 1.764 kg. of moist granules, corresponding to 263kg/hour, were obtained.

38.3 Weight percent of sodium phosphate solution, corresponding to 19.8weight percent of sodium tripolyphosphate, were required to be used,based on the material to undergo granulation. I

The granules were dried and sieved and the following fractions wereobtained.

Primary particles: Total yield of particles:

63 weight percent 75 weight percent.

The granules were annealed at 540 to 560C and a final product having thefollowing properties was obtained:

Phase-I: 76 weight percent Abrasion:

Drum test: residue on 0.4 mm sieve: 47 weight percent Sieve test:residue on 0.4 mm sieve: 89.5 weight percent Rate of hydration(ROH-test): after 1 minute: 94.6C

after 2% minutes: 94.8C (max. temperature) Apparent density: 894grams/liter.

EXAMPLE 3 a 0.3 mm 0.7 weight percent |0.l weight percent 0.2 mm 5.3weight percent 22.9 weight percent 0.1 mm 22.1 weight percent 39.9weight percent 0.06 mm 34.5 weight percent 48.9 weight percent 0.03 mm55.2 weight percent 69.1 weight percent 0.03 mm 44.8 weight percent 30.9weight percent The granulating disc was the same as that used in Example1.

The nozzle had on orifice of 1.6 mm in diameter. 1.288 kg/minute 0.795liter/minute was put through over a period of 6 hours and 48 minutes.The quantity of material put through within that period of timeaccordingly was 1.533 kg of sodium tripolyphosphate with 25 percent ofreturn material and 526 kg of sodium phosphate solution, and 2.059 kg ofmoist granules, corresponding to 303 kg/hour, were obtained.

34.3 Weight percent of sodium phosphate solution were required to beused, based on the material to undergo granulation. This corresponded to17.7 weight percent of solid tripolyphosphate.

The granules were dried and sieved, and the following fractions wereobtained:

weight percent 85 weight percent.

Primary particles: Total yield of particles:

The fraction consisting of particles with a size of between 0.4 and 1.5mm was annealed over three different periods of time in a rotary tube ata granulation temperature of between 540 and 560C.

RESULTS a. Throughput 84 kg/hour. The rotary tube was fed with 40 kg ofmaterial. This gave a sojourn time of 28 minutes and 30 seconds.

Phase-l: Abrasion: Drum test:

Apparent density: 949 grams/liter. b. Throughput 108 kg/hour; sojourntime 22 minutes and 18 seconds.

Phase-l: Abrasion: Drum test:

85 weight percent residue on 0.4 mm sieve: 71 weight percent residue on0.4 mm sieve: 96.5 weight percent after 1 minute:

Sieve test:

Rate of hydration 955C (ROH-test) after 2% minutes: 95.9C (max.temperature) Apparent density: 940 grams/liter. c. Throughput 144kg/hour; sojourn time 16 minutes and 42 seconds.

Phase-lz Abrasion: Drum test:

weight percent residue on 0.4 mm sieve: 65.2 weight percent residue on0.4 mm sieve: 94.6 weight percent after 1 minute:

Sieve test:

Rate of hydration 916C (ROH-test) after 2% minutes: 92.1C

(max. temperature) Apparent density: 942 grams/liter.

We claim:

1. 1n the process for granulating finely divided alkali metaltripolyphosphate by mixing the said tripolyphosphate with alkali metalorthophosphates and subjecting the resulting product to temperaturesufficient to effect the conversion of the alkali metal orthophosphatesto alkali metal tripolyphosphate the improvement comprising the steps ofa. charging an alkali metal tripolyphosphate of which between 10 and 40percent of its particles have a size smaller than 0.03 mm, the balanceof the particles having a size between 0.03 and 0.4 mm b. keeping inrotatory motion the said tripolyphosphate while spraying thereonto thesaid orthophosphates in the form of a solution with the resultantformation of shaped moist granules c. then drying the said moistgranules by trundling it through a heated rotating inclined tube d.thereafter separating undersize and oversize particle fractions from thedried granules and e. finally annealing the remaining desired fractionof particles at temperatures between 300 and 600C whereby a granulatealkali metal tripolyphosphate of high abrasion resistance and anapparent density of more than 750 g/l is obtained.

2. The process as claimed in claim 1, wherein between 20 and 30 percentof the alkali metal tripolyphosphate particles have a size of less than0.03 mm, the balance particles having a size of between 0.03 and 0.4 mm.

3. The process as claimed in claim 1, wherein the alkali metalorthophosphate solution used to effect granulation contains alkali metaloxide] P in a molar ratio of approximately 1.66.

4. The process as claimed in claim 1, wherein the alkali metalorthophosphate solution used for granulation contains approximately 50weight percent of dissolved phosphate.

5. The process as claimed in claim 1, wherein the a1 kali metalorthophosphate solution used for granulation is used at a temperature ofapproximately 90C.

6. The process as claimed in claim 1, wherein undersize and oversizeparticles are separated from the fraction of particles desired to beproduced, the oversize particles are crushed, particles of desirablesize are separated therefrom, and the two fractions of undersizeparticles are combined with starting material.

7. The process as claimed in claim I, wherein a concentration of between50 and 100 percent of phase-I is established by effecting the annealingstep over a period of between about and minutes at temperatures ofbetween about 520 and 550C.

8. The process as claimed in claim 1, wherein a concentration of between0 and 50 percent of phase-I is established by effecting the annealingstep over a period of between about 10 and 30 minutes at temperatures ofbetween about 300 and 500C.

9. The process as claimed in claim 1, wherein the hot granules arequenched and successively cooled down to room temperature, to stabilizethe phase-concentration therein.

2. The process as claimed in claim 1, wherein between 20 and 30 percentof the alkali metal tripolyphosphate particles have a size of less than0.03 mm, the balance particles having a size of between 0.03 and 0.4 mm.3. The process as claimed in claim 1, wherein the alkali metalorthophosphate solution used to effect granulation contains alkali metaloxide / P2O5 in a molar ratio of approximately 1.66.
 4. The process asclaimed in claim 1, wherein the alkali metal orthophosphate solutionused for granulation contains approximately 50 weight percent ofdissolved phosphate.
 5. The process as claimed in claim 1, wherein thealkali metal orthophosphate solution used for granulation is used at atemperature of approximately 90*C.
 6. The process as claimed in claim 1,wherein undersize and oversize particles are separated from the fractionof particles desired to be produced, the oversize particles are crushed,particles of desirable size are separated therefrom, and the twofractions of undersize particles are combined with starting material. 7.The process as claimed in claim 1, wherein a concentration of between 50and 100 percent of phase-I is established by effecting the annealingstep over a period of between about 15 and 30 minutes at temperatures ofbetween about 520* and 550*C.
 8. The process as claimed in claim 1,wherein a concentration of between 0 and 50 percent of phase-I isestablished by effecting the annealing step over a period of betweenabout 10 and 30 minutes at temperatures of between about 300* and 500*C.9. The process as claimed in claim 1, wherein the hot granules arequenched and successively cooled down to room temperature, to stabilizethe phase-concentration therein.